In PARS, do I have to start from scratch each time or can I copy an activity?

You can indeed copy an activity from the activity summary view in PARS.  Just hover your mouse over the activity and then click on the “copy” option.  You can also copy an activity from the activity detail view by clicking the “copy” option on the right side of the page.  This guide provides additional details, including which fields will get copied and which will not.